1. Field of the Invention
This invention relates to an ion beam deposition apparatus, and more particularly to an ion beam deposition apparatus which is adapted to impart kinetic energy to a vaporized material ionized in an ionization region and transport the ionized vaporized material to a substrate together with non-ionized vaporized material to uniformly deposit the materials on the substrate.
2. Description of the Prior Art
A vapor deposition film has been conventionally prepared using ion plating techniques, ion beam deposition techniques or cluster ion beam deposition techniques. Such deposition techniques each are to ionize a material of which the vapor deposition film is to be formed and apply acceleration voltage to the ionized material or utilize the charge effect of the ionized material as well as the application of the voltage to impart kinetic energy thereto, to thereby control the crystallizability and other physical properties of the deposited film.
In the conventional deposition techniques, it is required to provide an accelerating electrode adjacent to the crucible which serves to accelerate a vaporized material ejected from a crucible and ionized by the collision with an electron shower.
For example, the ion beam deposition techniques are practiced using such an apparatus as shown in FIG. 1. More particularly, a material 2 to be deposited is charged in a crucible 1 and then the crucible is heated. The heating of the crucible may be carried out by a radiation heating method using radiation heat generated from a crucible heating filament 3, an electron impact type heating method using electrons emitted from the filament 3 by applying voltage between the filament 3 and the crucible 1, or a direct heating method wherein electrical current is flowed directly through the outer surface of the crucible 1. Alternatively, the heating may be carried out by any combination of the above-mentioned heating methods. The heating of the crucible 1 causes the material to be changed to a vapor 4, which usually comprises microaggregates constituted by several molecules or atoms.
Then, an electron shower 5 is entangled in or collided with the vapor 4 to ionize a part of the vapor. The electron shower 5 is formed by cooperation of a filament 6 for emitting ionization electrons and a mesh-like grid 7 for deriving the electrons from the filament 5. In order that the electron shower 5 may be effectively directed toward the vapor 4, the filament 6 is conventionally surrounded by a shielding electrode 8 having the same potential as the filament 6 and the grid 7 is applied thereto voltage of 200 V to 1 kV positive with respect to the filament 6 and shielding electrode 8. Such construction is applied to the positive ionization of atoms or molecules due to ionization. The filament 6, grid 7 and shielding electrode 8 constitute an ionization electrode group, and the collision of the electron shower 5 with the vapor 4 upward ejected from the crucible 1 is carried out in an ionization region 9.
The particles of the ionized vapor are positively charged. Accordingly, in order to accelerate the ionized particles to impart kinetic energy thereto, it is required to provide an accelerating electrode 10 to apply negative acceleration voltage of 0-10 kV.
Equipotential lines 11 of the applied voltage, as shown in FIG. 1, are distributed in a concave form within the ionization region 9 when viewing from the upper side and the ionized particles are gradually accelerated while being applied thereto a force perpendicular to the equipotential lines 11 so that the ionized particles are converged about the central axis. FIG. 1 also shows that the equipotential lines 11 existing at the upper area of the ionization region are distributed in a convex form to be diverged. However, the particles are transported at a high velocity through the area because of having been already accelerated. Thus, the equipotential lines are converged. Further, high negative voltage applied or voltage optimum for forming a deposited film which is determined depending upon the applications causes a degree of convergence to be significantly varied.
A part of the vaporized particles which has not been ionized in the ionization region 9 reaches a substrate 12 for deposition without being accelerated and converged.
This causes the ionized and accelerated particles to be predominantly distributed on the central region of the substrate 12 and inferiorly distributed on the periphery thereof, resulting in a film deposited on the substrate being non-uniform.
Such phenomenon appears in not only the ion beam deposition techniques using such an apparatus as shown in FIG. 1 and utilizing the ionized molecule or arom-like particles but also cluster ion beam deposition techniques utilizing atom aggregates each formed of about 500-2000 atoms loosely bonded together.
The cluster ion beam deposition techniques will be hereinafter described in connection with an apparatus shown in FIG. 2.
The cluster ion beam deposition techniques utilize adiabatic expansion due to ejection. For this purpose, a material 16 to be deposited is charged in a crucible 15 provided with an injection nozzle 14. The crucible 15 is heated by a heating method of the electron impact type using electrons emitted from a filament 17 to vaporize the material 16. Alternatively, the heating of the crucible 15 may be carried out by a direct heating method utilizing large current flowed through the wall of the crucible 15, a heating method by means of a heater disposed around the crucible 15 or a combination thereof.
When the material 16 which has been vaporized in the crucible 15 to form a vapor having high pressure of the order of 10.sup.-2 to several Torr is ejected from the crucible 16 through the nozzle 16 to a high vacuum region, it forms atom aggregates each formed of 500-2000 atoms or clusters by supercooling due to adiabatic expansion and is transported toward a substrate in the form of a cluster beam 18 with kinetic energy imparted thereto at the time of the ejection from the nozzle 14. In the course of the ejection, an electron shower is formed by cooperation of a filament 19 for emitting electrons necessary for ionization of the clusters and positive voltage of about 200 V-1 kV which is applied to a mesh-like grid 20 for deriving electrons from the filament 19, as in FIG. 1. The electron shower thus formed is effectively irradiated on the cluster stream 18 with the assistance of a shielding electrode having the same potential as the filament 19 to ionize a part of the clusters. Such ionization is carried out in an ionization region 22. In FIG. 2, reference numerals 23, 24 and 25 designate an accelerating electrode, a substrate holder and a substrate, respectively. The apparatus shown in FIG. 1 may be provided with a thermo-couple 26, a heater 27 for heating the substrate 25 and a shutter 28, as desired depending upon the applications.
In such a case, the ionization of each of the clusters is carried out with respect to at least one of 500-2000 atoms constituting the cluster. In order to accelerate the ionized clusters, an electrical field produced by the accelerating electrode 23 enters or penetrates into the ionization region 22 to form a convergent lens system. This is accomplished in the substantially same manner as in FIG. 1. This causes the ionized clusters to be converged or focused about the central axis by the acceleration voltage. Thus, a film deposited on the substrate 25 supported on the substrate holder 24 is formed of clusters ionized in the ionization region 22 and subjected to the converging or focusing action and neutral clusters non-ionized and straightly transported to the substrate 25 which are non-uniformly distributed to each other. The distribution of the ionized clusters and non-ionized clusters is varied every variation in acceleration voltage depending upon the applications.
FIG. 3 illustrates examples of a computer simulation of such phenomenon in the apparatus of FIG. 2. The computer simulation of FIG. 3 was obtained using clusters of silver, wherein the portion from the ionization region 22 to the substrate 25 in FIG. 2 is enlargedly shown.
In the apparatus shown in FIG. 2, it is required to apply electron deriving voltage of 200 V-1 kV between the ionization electron deriving grid 20 and the ionization electrode emitting filament 19. In light of this respect, in FIG. 3, the electron deriving voltage is set to be 500 V, voltage to be applied to an ionization electron deriving grid 20 corresponding to the grid 20 in FIG. 2 is set to be 0 V and voltage to be applied to an ionization electron emitting filament 19 corresponding to the filament 19 in FIG. 3 is set to be -500 V.
FIG. 3A shows trails of the ionized clusters obtained by applying voltage of -3 kV to an accelerating electrode 23 in FIG. 3 corresponding to the electrode 23 in FIG. 2, and FIG. 3B shows those obtained by applying voltage of -6 kV thereto.
As can be seen from FIGS. 3A and 3B, the distribution of the ionized clusters on a substrate 25 is significantly varied depending upon voltage applied to the accelerating electrode 23 in FIG. 3.
As described above in connection with FIG. 2, the penetration of the electrical field of the acceleration voltage into the ionization region 23 causes the convergence of the clusters. However, in the simulation shown in FIG. 3, particularly, FIG. 3A, the ionized clusters are not sufficiently converged. The reason seems to be as follows:
A program used for the simulation shown in FIG. 3 is prepared using the latest and extremely high techniques and is highly different from a program for a simulation prepared in view of only the divergence and convergence of clusters due to the electrical field of acceleration voltage. Supposing that a conventional program is used which is prepared in view of only the electrical field, the simulations shown in FIGS. 3A and 3B respectively have trails of ionized clusters as shown in FIGS. 3C and 3D which are converged and then diverged.
In the preparation of the program used for the simulations shown in FIGS. 3A and 3B, the space charge effect of the ionized clusters has been carefully considered as well. Thus, the trails of the ionized clusters in the simulations shown in FIGS. 3A and 3B are converged in view of the repelling action between the ionized clusters and the micro variation in space potential due to the charge of the ionized clusters. Accordingly, the simulations of FIGS. 3A and 3B are highly close to the real state. This indicates that the trails of the ionized clusters can be pursued even when the variation in ionization current causes the variation in amount of ionized particles.
However, neutral clusters which have not been subjected to ionization are not affected by the acceleration voltage. Thus, the proportion between the neutral clusters and the ionized clusters distributed on the substrate is non-uniform. This adversely affects the crystallizability and physical properties of a film deposited on the substrate.